virtio-scsi: don't waste CPU polling the event virtqueue
The virtio-scsi event virtqueue is not emptied by its handler function. This is typical for rx virtqueues where the device uses buffers when some event occurs (e.g. a packet is received, an error condition happens, etc). Polling non-empty virtqueues wastes CPU cycles. We are not waiting for new buffers to become available, we are waiting for an event to occur, so it's a misuse of CPU resources to poll for buffers. Introduce the new virtio_queue_aio_attach_host_notifier_no_poll() API, which is identical to virtio_queue_aio_attach_host_notifier() except that it does not poll the virtqueue. Before this patch the following command-line consumed 100% CPU in the IOThread polling and calling virtio_scsi_handle_event(): $ qemu-system-x86_64 -M accel=kvm -m 1G -cpu host \ --object iothread,id=iothread0 \ --device virtio-scsi-pci,iothread=iothread0 \ --blockdev file,filename=test.img,aio=native,cache.direct=on,node-name=drive0 \ --device scsi-hd,drive=drive0 After this patch CPU is no longer wasted. Reported-by: Nir Soffer <nsoffer@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Tested-by: Nir Soffer <nsoffer@redhat.com> Message-id: 20220427143541.119567-3-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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@ -138,7 +138,7 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
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aio_context_acquire(s->ctx);
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aio_context_acquire(s->ctx);
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virtio_queue_aio_attach_host_notifier(vs->ctrl_vq, s->ctx);
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virtio_queue_aio_attach_host_notifier(vs->ctrl_vq, s->ctx);
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virtio_queue_aio_attach_host_notifier(vs->event_vq, s->ctx);
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virtio_queue_aio_attach_host_notifier_no_poll(vs->event_vq, s->ctx);
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for (i = 0; i < vs->conf.num_queues; i++) {
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for (i = 0; i < vs->conf.num_queues; i++) {
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virtio_queue_aio_attach_host_notifier(vs->cmd_vqs[i], s->ctx);
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virtio_queue_aio_attach_host_notifier(vs->cmd_vqs[i], s->ctx);
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@ -3534,6 +3534,19 @@ void virtio_queue_aio_attach_host_notifier(VirtQueue *vq, AioContext *ctx)
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virtio_queue_host_notifier_aio_poll_end);
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virtio_queue_host_notifier_aio_poll_end);
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}
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}
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/*
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* Same as virtio_queue_aio_attach_host_notifier() but without polling. Use
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* this for rx virtqueues and similar cases where the virtqueue handler
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* function does not pop all elements. When the virtqueue is left non-empty
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* polling consumes CPU cycles and should not be used.
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*/
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void virtio_queue_aio_attach_host_notifier_no_poll(VirtQueue *vq, AioContext *ctx)
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{
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aio_set_event_notifier(ctx, &vq->host_notifier, true,
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virtio_queue_host_notifier_read,
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NULL, NULL);
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}
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void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx)
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void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx)
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{
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{
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aio_set_event_notifier(ctx, &vq->host_notifier, true, NULL, NULL, NULL);
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aio_set_event_notifier(ctx, &vq->host_notifier, true, NULL, NULL, NULL);
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@ -317,6 +317,7 @@ EventNotifier *virtio_queue_get_host_notifier(VirtQueue *vq);
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void virtio_queue_set_host_notifier_enabled(VirtQueue *vq, bool enabled);
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void virtio_queue_set_host_notifier_enabled(VirtQueue *vq, bool enabled);
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void virtio_queue_host_notifier_read(EventNotifier *n);
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void virtio_queue_host_notifier_read(EventNotifier *n);
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void virtio_queue_aio_attach_host_notifier(VirtQueue *vq, AioContext *ctx);
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void virtio_queue_aio_attach_host_notifier(VirtQueue *vq, AioContext *ctx);
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void virtio_queue_aio_attach_host_notifier_no_poll(VirtQueue *vq, AioContext *ctx);
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void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx);
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void virtio_queue_aio_detach_host_notifier(VirtQueue *vq, AioContext *ctx);
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VirtQueue *virtio_vector_first_queue(VirtIODevice *vdev, uint16_t vector);
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VirtQueue *virtio_vector_first_queue(VirtIODevice *vdev, uint16_t vector);
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VirtQueue *virtio_vector_next_queue(VirtQueue *vq);
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VirtQueue *virtio_vector_next_queue(VirtQueue *vq);
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